This invention relates to the hydroformylation of unsaturated compounds.
The hydroformylation of ethylenically unsaturated compounds is a well known process of considerable industrial importance. Generally, hydroformylation reaction involve the reaction of carbon monoxide and hydrogen in the presence of a catalyst with the double bond of a compound C.sub.1 .dbd.C.sub.2 to form a compound containing one carbon atom more than the starting material. The product usually contains an aldehyde group but may form other products such as alcohols, or a mixture of aldehydes and alcohols depending upon the catalyst and reaction conditions such as the hydrogen/carbon monoxide ratio.
The aldehyde groups or alcohol groups (--CH.sub.2 OH), may be linked to either C.sub.1 or to C.sub.2. If the groups bound to the free valencies of C.sub.1 are the same as the groups bound to the free valencies of C.sub.2 a single hydroformylated product will generally be formed. However, if different groups are attached to C.sub.1 and C.sub.2, the reaction product can be a mixture of different hydroformylated products, i.e. a mixture of different aldehydes and/or different alcohols. For example, starting from methylacrylate, hydroformylation might be expected to yield a mixture of methyl .alpha.-formylpropionate and methyl .alpha.-formylpropionate.
Electronic and/or stearic factors of the reactants, catalysts, and associated ligands can have a pronounced effect on the distribution of hydroformylation products. That is, some reaction products are generally impeded or prevented from forming in favor of others. Thus, notwithstanding that different groups are attached to the free valencies of C.sub.1 and C.sub.2, a single hydroformylated product generally predominates. In the case of the hydroformylation of methacrylic acid, for example, .beta.-formylmethacrylic acid, rather than .alpha.-formylmethacrylic acid, is generally the predominant hydroformylated product.
In many instances, it would be desirable if the .alpha.-formyl product could be selectively obtained. The formation of .alpha.-formylcarbonyl compounds is particularly interesting given their utility as intermediates for the production of specialty chemicals such as malonates of lactones and for pharmaceutical compounds. For example, U.S. Pat. No. 4,001,336 to Hoffman discloses the use of .alpha.-formylpropionic acid methyl ester in the production of 2,6,10-trimethyl-dodecan-1-al, an artificial flavoring.
Accordingly, attempts have been made to develop suitable catalyst systems and to establish the reaction conditions that favor the formation of the .alpha.-formyl products. Unfortunately, even where desired hydroformylation product have been formed with adequate selectivity, the reaction rates have been disappointingly low. Raising the reaction temperature has been considered as a means for increasing the rate but the stability of the catalyst is adversely affected at temperatures needed to meaningfully increase the reaction rate. Moreover, raising the reaction temperature proliferates undesired reactions such as double bond hydrogenation and polymerization at the expense of the selectivity to the desired hydroformylated compound.
M. Tanaka et al. in Bull. Chem. Soc. Japan, vol 50 (9), pp. 2351-2357 (1977) have published results of the hydroformylation of .alpha.-.beta.-unsaturated esters. They indicated the preferred conditions and catalyst systems for the selective formation of some .alpha.-formylated products. The Authors reported attaining selectivities of 95% and greater of the ethyl-.alpha.-formylpropionate product from the hydroformylation of ethylacrylate results in selective formation. However, this figure only denotes the proportion of .alpha.-formylpropionate related to the total amount of hydroformylated products. Thus, it ignores the formation of considerable amounts of hydrogenated product (ethylpropionate). Inclusion of these and other hydroformulated products would certainly bring the selectivity figures well below the reported 95% selectivity. Moreover, the highest selectivity obtained in hydroformylating an .alpha.-substituted, .alpha.-.beta.-unsaturated ester using a catalyst containing rhodium and 1,3-bis(diphenylphosphino) propane as ligand, was 81.4%. This too ignored the formation of saturated byproduct.
While the nature of the rhodium based catalysts themselves were addressed, perhaps the most important proposition preferred in the article by Tanaka et. al. was the relationship between ligand configuration and catalytic activity. The authors took the position that there is an optimum carbon chain length of about C.sub.2 to C.sub.4 between ligand phosphine moieties for increasing alpha selectivity of hydroformylation products. Interestingly, the authors selected only diphosphines substituted with bulky aryl or cyclic substituents. 1,2-bis(diphenylphosphino)ethane appeared to be most preferred.
The prominence of phosphine derivatives substituted with numerous bulky aryl or cyclic substituents has continued to this day. For example, EP 0375573 to Devon et al, selected ligands such as .alpha.,.alpha.-bis(diphenylphosphino)-o-xylene for use in conjunction with a rhodium based catalyst in the hydroformylation of lower olefins. U.S. Pat. No. 4,201,728 to Hughes employs ligands such as trans 1,2-bis(diphenylphosphinomethyl) cyclobutane to increase alpha selectivity of aldehyde hydroformylation products. Japanese Patent JP 1040-434-A to Kuraray disclosed the use of a tris (substituted phenyl) phosphite with a rhodium compound for the hydroformylation of vinyl compounds.
Throughout the prior art, alpha selectivity of the products of olefinic hydroformylation is wildly divergent when such ligands are present. None of the art cites alpha selectivities consistently greater than 75% (based on the total quantity of reaction products) across a range of proposed ligands and reaction conditions for the production of a given olefinic hydroformylation product. Indeed, it is more often the case that alpha selectivities below 50% (based on the total quantity of reaction products) are found among a class of ligands claimed in the production of a given olefinic hydroformylation product.
Hence, the need to define a hydroformylation process which consistently attains highly selective .alpha.-formylated products has not abated.